1. Field of Invention
The invention relates to a substrate for mounting components of electronic circuitry, such as integrated circuit electronic components and discrete electronic components. In particular, the invention relates to metal-ceramic substrates that are suitable for mounting components of electronic circuitry used in applications such as control and conversion of electric power.
2. Description of the Related Art
Substrates comprising a ceramic layer and a metal layer are commonly used as devices for mounting electronic components to provide electrical isolation and high current carrying capacity. Such mounting devices commonly have thermal conductivity properties that allow them to transfer heat generated by electronic components away from the electronic components to a heat sink. Mounting devices with appropriate thermal conductivity properties reduces the power consumption of electronic components, allowing electronic circuit designers to use electronic components with reduced footprints. This is particularly important in certain military and space applications, where space may be limited and lower weight electronic components are desired.
Examples of such substrates use a copper-copper oxide eutectic bond between copper and aluminum oxide. These substrates are commonly referred to as Direct Bond Copper (DBC) device because the bonding process does not require a filler metal. For example, U.S. Pat. No. 3,911,553 describes such a method of creating a DBC-type substrate.
However, DBC-aluminum oxide mounting devices do not have the thermal conductivity necessary to allow them to transfer heat away from the electronic components, which is important for high power and high temperature applications such as military, space or automotive applications. Therefore, DBC-aluminum nitride mounting devices have been preferred due to their superior thermal conductivity. For example, U.S. Pat. No. 4,996,116 describes an enhanced direct bond structure of copper and aluminum nitride for mounting electronic components.
Although DBC-aluminum nitride devices are more effective than DBC-aluminum oxide devices in transferring heat away from the electronic components, DBC-aluminum nitride devices still present several disadvantages.
For example, the eutectic bond between copper and aluminum nitride commonly fails during the thermal cycling that is required for certain applications, such as military and space applications. Military and space applications typically require electronic components that can survive approximately 1000 thermal cycles from xe2x88x9265xc2x0 C. to 200xc2x0 C. However, most DBC-type devices fail well below the required 1000 thermal cycles.
One of the reasons for this failure is the extreme difference in the co-efficient of thermal expansion (CTE) between the copper layer of DBC-type devices and the ceramic layer. For example, the copper layer of a DBC-type device has a CTE of approximately 18 ppm/xc2x0 C., whereas the aluminum nitride layer has a CTE of approximately 5 ppm/xc2x0 C. Because of this extreme difference in CTE, the copper layer has a tendency to harden during thermal cycling, resulting in the ceramic cracking.
The CTE of the copper layer of DBC-type mounting devices is also extremely different from that of materials commonly used to make electronic components, such as silicon, silicon carbide and gallium arsenate. For example, silicon has a,CTE of approximately 2.8 ppm/xc2x0 C., which is much lower than the CTE of the copper layer of DBC-type mounting -devices, which is approximately 18 ppm/xc2x0 C. Due to this extreme difference in CTE, the copper layer of the DBC-type device tends to contract more than the electronic components mounted to it, warping the electronic components out of their original shapes, which in turn affects the performance of the electronic components.
Another disadvantage of the DBC-type devices is their inability to be reheated to temperatures above approximately 650xc2x0 C. in a reducing atmosphere without separating the copper-ceramic bond. Reheating is sometimes necessary to mount additional electronic components onto the mounting device or to join the mounting device carrying the electronic components to a base plate or a heat sink. For example, the process of brazing the DBC-type substrate to a base plate may require the mounting device to be reheated up to 700xc2x0 C. in a reducing atmosphere, a temperature that delaminates a DBC-ceramic bond.
Another problem with the DBC-type devices occurs when creating a eutectic bond between copper and a ceramic. Successful creation of such a eutectic bond is dependent on accurately maintaining furnace parameters such as furnace temperature, furnace atmosphere and time at desired temperature during the bonding process. Even slight changes in these furnace parameters can cause an incomplete eutectic melt formation, resulting in an inadequate bond.
Furthermore, the process of direct bonding copper to ceramic creates a thin layer of oxidation on the copper surface for mounting electronic components, which is not desirable, especially if the mounting surface has to be rebrazed to mount additional electronic components. This requires the layer of oxidation to be removed before the rebrazing process, adding an additional level of unnecessary complexity.
Accordingly, there is a need in the art for a substrate-type device for mounting electronic components, which provides superior thermal conductivity and is also capable of surviving thermal cycles required for high power or high temperature applications, such as military, space or automobile applications. There is also a need for a mounting device that can be reheated to brazing temperatures without weakening the metal to ceramic bond.
The present invention relates to devices for mounting electronic components to form part of electronic circuitry. The devices preferably comprise two metal members coupled to one ceramic member with the resulting assembly having two major surfaces comprising the metal members and the ceramic member disposed in-between. One of the metal members will then be used to mount electronic components. The other metal member will be joined to either a base plate or directly to a heat sink forming part of a hybrid electronic circuit.
In one embodiment of the invention, the second metal member is mounted onto a base plate. In this embodiment, the metal members are preferably formed using a copper-molybdenum-copper (CMC) laminate comprising one layer of molybdenum disposed between two layers of copper, with one surface of each of the CMC laminates being joined to opposite major surfaces of the ceramic. In this embodiment, the ceramic comprises aluminum nitride, which has suitable thermal conductivity for high temperature applications. However, other ceramics such as aluminum oxide, silicon nitride, or beryllium oxide are acceptable alternatives.
According to various embodiments of this invention, the CTE of the CMC laminates is similar to the CTE of the ceramic being joined between each CMC laminate. This will allow the joint between the CMC laminate and the ceramic member to survive successive thermal cycling. Therefore, the CTE of a CMC is preferably varied to match the CTE of the ceramic member.
The CTE of the CMC laminate is varied by changing the thickness of each of the two copper layers and the molybdenum layer relative to the entire thickness of the CMC laminate. For example, for an aluminum nitride device, the thickness of the various layers comprising the CMC laminate will be selected to achieve a CMC laminate CTE similar to the CTE of aluminum nitride. However, if the ceramic chosen is aluminum oxide, then the thickness of the various component layers of the CMC laminate will be adjusted to achieve a laminate CTE similar to the CTE of aluminum oxide. It is also possible to manufacture the metal members of the mounting device from metal composites such as copper-molybdenum composite, copper-tungsten composite, or laminates of metals other than copper and molybdenum.
In another embodiment of the invention, the second metal member itself serves as the base plate, which is mounted to a heat sink to form part of a hybrid electronic circuit. In this embodiment, the metal members are preferably formed as described above. In some instances, this structure provides an advantage over the first described embodiment because it reduces the number of interfaces through which the heat generated by the electronic components has to travel in order to reach the heat sink. In a high power or high temperature application, this can improve the performance of the electronic components. However, it may be necessary that the base plate comprise metal substrates other than a CMC laminate due to manufacturing specifications provided by a circuit designer for reasons peculiar to the specific application. For example, eight restrictions might influence the choice of the base plate material.
The present invention also relates to a method of making devices for mounting electronic components. More specifically, the two CMC laminates for forming the two metal members of the present mounting device are prepared by joining a layer of molybdenum with two layers of copper on each of its two major surfaces. Both the CMC laminates are then coated on one surface with an active metal, such as titanium, preferably (although not necessarily) using a sputtering process or other suitable physical vapor deposition processes. The surfaces of the CMC laminates coated with an active metal, such as titanium, are then brazed onto a ceramic member, such as aluminum nitride.
The brazing is accomplished by heating the assembly of the two CMC laminates coated with an active metal, such as titanium and the ceramic member, above a point sufficient to create an initial melt formation between the active metal and the copper layer of the CMC laminate, but below the melting point of the copper. Once this assembly is cooled, the alloy resulting from the melt formation between the active metal and the copper layer of the CMC laminate forms a firm bond between the CMC laminate and the ceramic member. In one embodiment, one surface of the assembly is mounted onto a separate base plate. In an alternate embodiment, the bottom CMC laminate can itself serve as the base plate. The top CMC laminate surface of the joined assembly can be used to mount the electronic components.
One of the advantages provided by this invention is that the CTE of the CMC laminate can be easily varied to match the CTE of the ceramic member. Such a match in the CTE will provide an assembly that can survive thermal cycling. Another advantage of this invention is that it allows the heat sink to draw heat away from electronic components more effectively, which reduces the failure of such components due to overheating. Furthermore, the mounting device of this invention can be more easily manufactured than DBC-type mounting devices. Another advantage of the metal-ceramic mounting device according to various embodiments of this invention is that it can be reheated to brazing temperatures without weakening the bond between the ceramic and the metal.